In image display devices such as cathode ray tube display device (CRT), plasma display panel (PDP) and liquid crystal display device (LCD), in order to prevent a deterioration in the contrast or reflection by the display due to reflection of external light, antireflection films are usually disposed on the outermost surface of the display as a film for reducing a reflectance based on the principle of optical interference.
As a method of forming antireflection films, generally known is a method of applying an optically functional layer to a transparent support. For the purpose of antireflection, a reflectance can be reduced, for example, by disposing a layer (low refractive index layer) having a lower refractive index than that of the transparent support, or disposing a high refractive index layer over the transparent support and then laying the low refractive index layer over the high refractive index layer. Antireflection films formed by such coating method are suited for mass production, because they can be produced continuously.
In antireflection films having, over a transparent support, only a layer (low refractive index layer) having a refractive index lower than that of the transparent support, however, the refractive index of the low refractive index layer must be reduced sufficiently in order to reduce the reflectance of the antireflection films. For example, in an antireflection film having triacetyl cellulose as a support and a UV curable film of dipentaerythritol hexaacrylate as a hard coat layer having a low refractive index, the refractive index must be reduced to 1.40 or less in order to adjust an average reflectance at 450 nm to 650 nm to 1.6% or less. As a material having a refractive index of 1.40 or less, magnesium fluoride and calcium fluoride can be given as examples of inorganic materials, while fluorine-containing compounds having a large fluorine content can be given as examples of organic materials. Since fluorine compounds lack cohesive power, a film available therefrom has not sufficient scratch resistance as a film to be disposed on the uppermost surface of a display. For the formation of a film having sufficient scratch resistance, therefore, a compound having a refractive index of 1.43 or greater is required.
The above-described problems can be overcome by increasing the refractive index of a layer underlying the low refractive index layer, that is, adopting a structure in which a high refractive index layer is disposed over a transparent support and a low refractive index layer is then laid over the high refractive index layer. For example, an antireflection film having a low refractive index layer disposed over a transparent support via a hard coat layer is described in Japanese Patent Laid-Open No. Hei 7-287102. Also described in it is that the reflectance can be reduced by increasing the refractive index of the hard coat layer.